Basically if you update you system to 4.0.0=< you'll update your game card slot controller's firmware permanently and you won't be able to use it on lower versions again.
I remember there is a .kip that disables the slot's functionality so it won't get updated but don't remember where you can find it.
If you care about being able to play your cartridges on 3.0.2=> so you should use that .kip file.
I think ReiNX by itself will update the gamecard slot but haven't looked into the source to confirm it.